My answer is by themselves; no, they cannot. The choices we make in life define who we will become, they define who we are. We as humans seem to become molded, and once we harden we must be completely remade. And God is the only one who can “remake” us. He is the potter, we are merely the clay. Who are we to tell our maker how we want to be shaped? Like a cup without a handle, we our selves, with out God are useless. But God wants to use us, shape us, he wants us to become his unique creation. He will never take over our lives, but he wants to be a part of us.  Yes, to a certain extent we ourselves can change, like a friend just brought to attention to me, someone without God could quit smoking.  But this is the outside of a person.  When it comes to the heart of someone; He is the one who changes us. Only with God can we change. He is the one who makes us. He is the way. He is the truth. He is the life. Thing is; some live without it.